1. Field of the Invention
The present invention relates to an optical transmission system and, more particularly, to an optical transmission system employing orthogonal polarization multiplexing in which mutually independent information is put onto orthogonally polarized waves, which are then polarization-multiplexed for transmission.
2. Description of the Related Art
Recent years have seen the commercialization of some optical transmission systems that can transmit large-volume signals of 1 Tb/s or more, per optical fiber core, over several hundred kilometers by wavelength-multiplexing a plurality of signal channels of 40 Gb/s per wavelength. Currently, earnest efforts are being made to develop technologies capable of realizing even wider bandwidths by raising the transmission rate per wavelength to 100 Gb/s.
To that end, electrical circuitry having wider bandwidth capacity must be introduced if higher transmission rates per wavelength are to be achieved. However, a problem with the current device technology is its inability to readily design electrical circuitry capable of realizing the transmission rate of 100 Gb/s.
Also, the optical signal-to-noise ratio (OSNR) required to achieve certain code error rates is inversely proportional to the bit rate. To realize 100 Gb/s, therefore, the optical signal-to-noise ratio must attain an improvement of about 10 dB over the transmission rate of 10 Gb/s. Yet, the resulting problem is the limited interval of optical amplifiers and the limited number of relay devices, which in turn will shorten the transmission distance.
Further, if the modulation bandwidth rises to 100 Gb/s, the spectrum width of the optical signal will be widened. Therefore, the wavelength interval needs to be widened if the crosstalk with neighboring wavelengths is to be avoided. And the consequence is the problem of lowered frequency usage efficiency.
As a way of overcoming this problem, orthogonal polarization multiplexing and digital coherent receiving have been the focuses of attention in recent years.
Orthogonal polarization multiplexing is a technique of multiplexing signal channels using polarized waves, which are one of the properties of light as electromagnetic waves. Light, which is transverse waves, has two polarization components orthogonal to its direction of travel. In orthogonal polarization multiplexing, mutually independent information is put onto these two polarized waves, and they are multiplexed by a polarization beam combiner for transmission. In this multiplexing method, two signals of 50 Gb/s, for instance, can be polarization-multiplexed for a transmission rate of 100 Gb/s per wavelength. Thus, it is possible to decrease the frequency bandwidth of electrical circuitry by half while maintaining the transmission rate per wavelength.
Next, digital coherent receiving is a method of using digital signal processing to the best advantage in coherent optical communications. The coherent optical communications are a technology which was subjected to active research and development (R&D) in the 1980s. In this technology, an optical signal transmitted over a long distance is made to interfere with an optical signal from a local oscillator (LO) installed at a receiving station, and the resulting beat signal is used in the receiving. It was expected then that high receiving sensitivity could be achieved by raising the power of the LO and long-haul and high-speed transmissions would be realized thereby. However, the practical application of this technology did not materialize because of the necessity to achieve exact agreement in transmission frequency and signal phase between the laser of a transmitting station installed in a remote location and an LO and also the failure to realize a high-precision laser to be used therein. In the meantime, the performance of the conventional intensity modulation-direct detection (IM-DD) method has undergone dramatic improvements due to the arrival of the erbium-doped fiber amplifier (EDFA) and wavelength division multiplexing (WDM). Consequently, the R&D of the coherent receiving has had to follow the path of decline.
However, the improvement of performance characteristics by EDFA and WDM is now approaching its limit now that the transmission rate per wavelength has risen as high as 100 Gb/s. Also, stabler interference can now be achieved by real-time compensation by digital signal processing for the above-mentioned errors resulting from disagreement in frequency and phase of laser beams. Under such circumstances, the coherent receiving method is again coming under the spotlight today. Another major factor in this development is the advance of CMOS device technology which is making the so-far unfeasible ultrahigh-speed electronic circuitry feasible. By making the most of the digital signal processing technology, the digital coherent receiving method can achieve, within the electrical domain, the splitting of polarization-multiplexed signals and compensation for wavelength dispersion or polarization mode dispersion. Therefore, this technology may present possibilities of significant improvements in optical transmission characteristics. For the orthogonal polarization multiplexing method and the digital coherent receiving method, see Reference (1) in the following Related Art List.
FIG. 1 is a block diagram showing an exemplary structure of a digital coherent optical transmission system. A digital coherent optical transmission system 110 shown in FIG. 1 includes a polarization multiplexing optical transmitter 120 and a polarization multiplexing optical receiver 130.
In the polarization multiplexing optical transmitter 120, continuous-wave (CW) light outputted from a continuous-wave (CW) laser 121 is split into two branches by a 3 dB coupler 122, and the two branches of light are inputted to a first Mach-Zehnder modulator 123 and a second Mach-Zehnder modulator 124, respectively. At the first Mach-Zehnder modulator 123, one of the CW light beams is externally modulated in accordance with a first client signal, whereas at the second Mach-Zehnder modulator 124, the other of the CW light beams is externally modulated in accordance with a second client signal. The optical signal outputted from the first Mach-Zehnder modulator 123 is inputted to the X-axis side of a polarization beam coupler (PBC) 125. The optical signal outputted from the second Mach-Zehnder modulator 124 is inputted to the Y-axis side of the polarization beam coupler 125 after passing through a ½ wavelength plate 126. Hereinafter, the optical signal inputted to the X-axis side of the polarization beam coupler 125 will be referred to as an X-polarized signal, and the optical signal inputted to the Y-axis side thereof as a Y-polarized signal. The X-polarized signal and the Y-polarized signal are then polarization-multiplexed by the polarization beam coupler 125 and outputted to an optical fiber transmission path 140 as a polarization-multiplexed optical signal.
In the optical fiber transmission path 140, the polarization state of the signal as it is outputted from the polarization multiplexing optical transmitter 120 is not preserved, but the polarization-multiplexed optical signal changes into various polarization states such as linearly-polarized light, right-handed or left-handed circularly-polarized light, and elliptically-polarized light as it propagates therethrough. The polarization state, which is not constant temporally either, keeps changing under the influence of various disturbances to the optical fiber transmission path 140.
In the polarization multiplexing optical receiver 130, the polarization-multiplexed optical signal from the optical fiber transmission path 140, having the two polarized waves mixed, is inputted to a polarization beam splitter (PBS) 131, where it is split into two polarized waves. Also, outputted from the local oscillator (LO) 132 is a local light having nearly identical wavelength to that of the signal light. This local light is separated into two polarization components by a polarization beam splitter 133. The four polarization components split by the polarization beam splitters 131 and 133 are inputted to optical circuits called optical 90-degree hybrid circuits 134 and 135, where the signal light and the local light are made to interfere with each other.
FIG. 2 illustrates an exemplary structure of optical 90-degree hybrid circuits 134 and 135. A received signal light is inputted to one of the input ports of the optical 90-degree hybrid circuit, and a local light is inputted to the other of the input ports thereof. The signal light is split into two beams by a coupler 210, and the local light is split into two beams by a coupler 212. One of the split signal light is coupled to the local light by a coupler 211 and then outputted to a balanced photodiode (PD) disposed posteriorly. The other of the split signal light is coupled by a coupler 214 to the local light, whose phase has been shifted by 90 degrees as it passed through a 90-degree phase shift circuit 213, and then outputted to a balanced photodiode (PD) disposed posteriorly.
Referring back to FIG. 1, the digital coherent optical transmission system 110 will further be explained. The optical signal received by the polarization multiplexing optical receiver 130 is split into two branches by the polarization beam splitter 131, and is further split into two branches by the optical 90-degree hybrid circuits 134 and 135, respectively. That is, the optical signal is split into a total of four optical signals by the optical circuit up to the optical 90-degree hybrid circuits 134 and 135. These four optical signals are subjected to optical-to-electrical (O/E) conversion by four balanced photodiodes 136.
The four electric signals outputted from the balanced photodiodes 136 are amplified by electric amplifiers 137 and then digitized by an ultrahigh-speed analog-to-digital converter (ADC) 138. The digitized signals are inputted to a digital signal processor (DSP) 139 where they are subjected to compensation for frequency/phase shift, between the received signal light and the local light, and polarization splitting. Further, waveform shaping, such as wavelength dispersion compensation and polarization mode dispersion compensation, is performed at the DSP 139. Then, through a clock extraction, the first client signal and the second client signal, which have been loaded on the X-polarized signal and the Y-polarized signal, respectively, by the polarization multiplexing optical transmitter 120, are reproduced (refer to Reference (2), in the following Related Art List, for methods of polarization multiplexing/demultiplexing and dispersion compensation by digital signal processing).
Note that the method of receiving two polarized waves in a mixed state and splitting them subsequently by a digital signal processing is called polarization diversity. Diversity, which signifies variety, is a technique widely employed to improve communication quality in the field of wireless communications. The polarization diversity in wireless communications is a technique of using a bidirectional antenna (dual-polarized diversity antenna) by which both of the polarized waves are received and coupled for output or one of them with stronger output is used.
Thus, the technology of combining the above-described orthogonal polarization multiplexing transmission and digital coherent receiving is today a focus of attention as a very effective technology that can double the bit rate per wavelength while maintaining the operation speed of electrical circuitry.